De Dietrich GT 125A
The De Dietrich GT 125A is a 5-section eutectic cast-iron 3-pass boiler rated 144 MBH gas input / 122 MBH output (1 GPH on oil) at a printed AFUE of 85.5% gas / 88% oil, with 1-1/4" supply and return connections.
Key figures
- Max input
- 144 MBH
- Max CSA output
- 122 MBH
- AFUE
- Gas 85.5% / Oil 88%
- Min input
- —
Input is gas energy consumed; output is heat delivered. Never size from input alone.
De Dietrich GT 125A in practice
The GT 125A is the 5-section size of the GT 120A series, a eutectic cast-iron, three-pass, low-temperature boiler documented in the GT-120A brochure. The printed table gives 144 MBH CSA gas input, 122 MBH CSA output, a 1 GPH No. 2 fuel oil input, 7.9 US gallons of water capacity and a dry weight of 412 lb. The line is approved for direct-vent, Type L (gas-oil) or Type B (gas only) natural-draft venting.
Supply and return are 1-1/4" threaded connections and the vent connection is 5". The boiler is rated ASME 60 psi water with an operating range of 86–194 °F and a 230 °F maximum allowable water temperature; the minimum safety relief capacity printed for this size is 135 MBH. De Dietrich's North American arm, DDR Americas Inc., is in Cambridge, Ontario and states it warehouses and assembles boilers for the USA and Canada.
Best suited to
- Homes and light-commercial buildings whose calculated load fits 122 MBH output
- Low-temperature hydronic systems that benefit from the eutectic cast iron's thermal-shock resistance
- Retrofits using a chimney with Type L or Type B venting
Applicability and support
- Model number
- GT 125A
- Canadian availability
- available
- Production status
- current
- ROMA supplies
- Confirmed at consultation
- ROMA services
- Confirmed at consultation
- Warranty
- Not verified
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Specifications
Published specifications
Values as published by De Dietrich. Units are shown with every value; notes flag conflicting sources or missing figures.
| Specification | Value | Note |
|---|---|---|
| Input | ||
| CSA gas input | 144 MBH | Printed in the GT 120A brochure technical specifications. |
| CSA No. 2 fuel oil input | 1 GPH | Printed in the GT 120A brochure technical specifications. |
| Output | ||
| CSA output (gas-oil) | 122 MBH | Printed in the GT 120A brochure; output based on thermal-efficiency test per ANSI Z21.13a/CSA 4.9a-2005. |
| Efficiency | ||
| AFUE (as printed) | Gas 85.5% / Oil 88% | Printed in the GT 120A brochure; the product page says 'Efficiency up to 88%'. |
| Venting and air | ||
| Boiler vent connection | 5 in | Printed in the GT 120A brochure; approved for direct-vent, Type L (gas-oil) or Type B (gas only) natural-draft venting. |
| Physical and connections | ||
| Cast-iron sections | 5 | Printed in the GT 120A brochure dimensions table. |
| Heating supply / return | 1-1/4" threaded outlet / 1-1/4" threaded return | Printed in the GT 120A brochure dimensions drawing. |
| Weight (dry) | 412 lb | Printed in the GT 120A brochure. |
| Water capacity | 7.9 US gal | Printed in the GT 120A brochure. |
| Minimum safety relief capacity | 135 MBH | Printed in the GT 120A brochure. |
| ASME MAWP (water) | 60 psi | Printed in the GT 120A brochure. |
| Operating limits and other | ||
| Operating temperature range | 86–194 °F | Printed in the GT 120A brochure; max allowable water temp 230 °F. |
| Fuel | Natural gas or propane; No. 2 oil firing documented | GT 120A brochure burner table prints '40 G200 SBT NG or LPG' gas burners and No. 2 oil burners; the product page labels it 'Residential Gas or Oil' and the series page describes 'oil/gas/propane firing'. |
Manuals
Alerts documented for this model
None documented yet.
Frequently asked questions
What is the output of the GT 125A?
The GT 120A brochure prints 144 MBH CSA gas input and 122 MBH CSA output for this size, with a 1 GPH No. 2 oil input.
What efficiency does the GT 125A have?
The brochure prints AFUE of 85.5% on gas and 88% on oil for the GT 120A line; the product page says 'Efficiency up to 88%'.
Can the GT 125A use propane?
The brochure's burner table lists '40 G200 SBT NG or LPG' gas burners for the GT 120A series, so natural gas and propane are both documented. Oil firing is also printed.
Questions about the De Dietrich GT 125A?
Describe what the boiler is doing and get a straight answer about the next step.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Compare and explore
- De Dietrich GT 123A vs GT 125AComparison
- De Dietrich GT 123ASame series
- De Dietrich GT 124ASame series
- De Dietrich GT 126ASame series
- De Dietrich GT 224ASame series
- De Dietrich GT 225ASame series
- De Dietrich GT 226ASame series
- De Dietrich GT 227ASame series
- De Dietrich GT 228ASame series
- De Dietrich GT 334ASame series
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- De Dietrich GT 336ASame series
- De Dietrich GT 337ASame series
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- GT Series series hub
- Buying guide
Sources and verification
- De Dietrich NA — GT-120A product page · accessed September 9, 2026
- De Dietrich NA — GT-220A product page · accessed September 9, 2026
- De Dietrich NA — GT-330A product page · accessed September 9, 2026
- DDR Americas — Cambridge, Ontario company page · accessed September 9, 2026
- De Dietrich — GT 120A brochure (E816558) technical specifications · accessed September 9, 2026
- De Dietrich — GT-120A Installation & Operation Manual (2-27-2018) · accessed September 9, 2026
- De Dietrich — GT-120A engineering submittal (GT 123A to 126A) · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.